The present invention relates to a resistance welding method (a lap resistance welding method such as a spot welding method) in which, for example, a strand composed of twisted wires of one electric cable and a strand composed of twisted wires of the other electric cable are put on one another and pressed between a pair of electrodes, and a large current is passed through the strands for a short time to utilize resistance heating caused by the current conduction to thereby perform resistance welding, and an apparatus for use in such a method.
As such a type of resistance apparatus, there is an apparatus of an AC thyristor system (a system in which thyristors are used as electric source switches for performing welding current regulation continuously by changing the firing phase of the thyristors) shown in FIG. 6(a). The resistance welding apparatus 1, which performs resistance welding on strands 31 and 31' put one on the other, as materials to be welded, of a pair of electrically insulating coated electric cables 30 and 30' (hereinafter simply referred to as "cables"), has a box-like apparatus body 2 which is substantially U-shaped in side view. A cable-setting jig 3 is disposed in the center of the apparatus body 2. An air cylinder 4 is attached to the upper front of the apparatus body 2. A pair of upper and lower electrodes 5A and 5B, which serve to pass a welding current through a welding portion of the strands 31 and 31' and also serve to apply a predetermined amount of pressure to the welding portion, are provided below the air cylinder 4, and the cable-setting jig 3 respectively.
The upper electrode 5A is connected to a piston rod 4a of the air cylinder 4 through an electrode holder 6 so as to move vertically. Further, the upper electrode 5A is also connected, through an ounce copper plate 8, to a welding transformer (electric source) 7 which serves to supply a welding current. Further, the lower electrode 5B is fixed to the center portion of the apparatus body 2 and connected to the welding transformer 7. The welding transformer 7 is connected to a welding timer 9 which serves to set the current value and current-conduction time of the welding current. An electromagnetic valve 4A of the air cylinder 4 is opened/closed on the basis of conduction-start and conduction-end signals obtained from the welding timer 9. Incidentally, each of the electrodes 5A and 5B is constituted by a columnar chromium-copper matter 5a and a rectangular tungsten tip 5b.
The step of performing resistance welding of the strands 31 and 31' of the pair of cables 30 and 30', which are put on one another by means of the AC thyristor system resistance welding apparatus 1 as shown in FIG. 7 will be described with reference to a flow chart shown in FIG. 8. First, after the exposed strands 31 and 31' of the pair of cables 30 and 30' are put in between the pair of electrodes 5A and 5B through the cable-setting jig 3, a start input switch 9A is turned on so that the welding timer 9 starts (step S1). As a result, the electromagnetic valve 4A connected to a not-shown compression air source is opened and the upper electrode 5A is moved down by the air cylinder 4. After completion of initial pressure application to the strands 31 and 31' between the pair of electrodes 5A and 5B (step S2), a welding current is passed between the pair of electrodes 5A and 5B alternately upward and downward by the welding transformer 7 (step S3). The welding current is passed for the current-conduction time which is set (fixed) in advance. Resistance heating caused by the conduction of the welding current is utilized so that the strands 31 and 31' are fused. After resistance welding, the current conduction is stopped (step S4). Then, cooling is performed while the pressure application state between the pair of electrodes 5A and 5B is held for a predetermined time (step S5). The operation for the steps S2 to S5 is carried out automatically under the sequence control of the welding timer 9. Then, when the pressurized state is canceled, the resistance welding is completed (step S6). Incidentally, a similar technique is disclosed in Japanese Patent Unexamined Publication No. Hei. 4-22584.
In the conventional resistance welding apparatus 1, however, the current-conduction time of the welding current was fixed to a predetermined value under the constant-current control by means of a combination of the welding transformer 7 and the welding timer 9. Accordingly, when the secondary resistance value of the pair of electrodes 5A and 5B changed, the amount of heating also changed. Accordingly, the welding state (welding quality) such as welding strength (adhesive force), or the like, of the strands 31 and 31' composed of twisted wires varied easily. That is, when the heating value is Q, the heating value Q is expressed by Q=I.sup.2 .multidot.R.multidot.T in which the current I is constant and the time T is constant. The resistance R varies because of variations due to oxidation resistance of the strands 31 and 31' each of which is constituted by a plurality of twisted wires, the change in resistance due to the change in shape of the electrodes 5A and 5B made from tungsten, or the like, variations in the state of oxide film on the bonded surface of the tungsten tip 5b in each of the electrodes 5A and 5B, variations in pressure applied between the electrodes 5A and 5B, etc. Accordingly, it was difficult to make welding quality stable.